Method and instrument for determining deformability and residual stresses in foundation soils



Oct. 13, 1970 METHOD AND INSTRUMENT FOR DETERMINING DEFORMAB ANDRESIDUAL STRESSES IN FOUNDATION SOILS Filed May 6, 1968 V N ILITY4.'Sheets--Sheet -M. c. MENDES'DA ROCHA L 3 Oct. 13, 1970 M. c. MENDESDA ROCHA F-TAL 3,533,233

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUALSTRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 2 Get. 13,1970 c MENDES DA ROCHA ETAL 3,533,283

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUALSTRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 5 I5 5Ilia. 1/ Elk" 13, 1970 M. c. MENDES DA ROCHA ETAL 3,533,283

METHOD AND INSTRUMENT FOR DETERMINING DEFORMABILITY AND RESIDUALSTRESSES IN FOUNDATION SOILS Filed May 6, 1968 4 Sheets-Sheet 4 II I!Patented Oct. 13, 1970 Int. CI. (301 3/08 US. CI. 73-94 Claims ABSTRACTOF THE DISCLOSURE A method and apparatus for measuring deformability andresidual stress in a foundation mass, wherein a single elongated slot isformed by cutting a plurality of contiguous narrow slots whereafter afiat jack is inserted into the elongated slot and applies measuredpressure against the faces bounding the slot whereby the faces undergodeformation which is measured by the jack.

BRIEF SUMMARY OF THE INVENTION This invention relates to methods andapparatus for measuring the deformability and residual stresses in afoundation mass.

It is increasingly recognized as very important to know thedeformability of foundation masses such as soils, rock masses, and thelike when designing large structures to be built on or within rockmasses, such as concrete dams, bridges, large buildings, tunnels,underground power stations, etc.

Very often the deformability of these foundations is determinedexclusively in laboratory tests of soil or rock samples. Recently,however, the need for in situ tests has been recognized, at least in thecase of large structures involving large volumes of foundation rocks, soas to secure representative results of the true conditions in theground, particularly as regards heterogeneity and cracks.

These in situ tests generally comprise excavating trenches or galleries,applying known forces on given areas of their walls and measuring theresulting displacements.

These forces are applied by means of rigid concrete slabs cast againstthe gallery wall, with which they are in contact in an area of theorder, for example, of 1 square meter. By means of jacks applied to theslabs, it is possible to obtain pressures representative of those thatwill be applied to the ground by the completed structure. Circular steelplates with diameters of 20 or 50 cm. can be used for the same purpose.

Nevertheless, the distribution in the ground of the forces transmittedby concrete slabs or steel plates is imperfectly known, and makes theinterpretation of the test results difiicult. In order to surmount thisdifiiculty, it is sought to obtain a distribution as uniform as possibleof the loads in the ground, for which rubber plates are sometimes usedbetween the loaded surface and the jacks.

A still more uniform distribution of pressure can be obtained by meansof oil-filled deformable flat jacks. These jacks are applied in narrowslots cut in the rock mass and usually cannot be reused because thespace between the jack and the slot is filled with mortar or concrete.

In deformability tests, strains are measured in the direction of theapplied forces sometimes at the center of,

' sometimes outside, the loaded areas.

From the above, it follows that the methods so far used in deformabilitytests of soils and rocks are very expensive and slow, due to the need todrive a gallery or cut a slot in the rock mass to be studied. Blastingmethods have the disadvantage of disturbing the structure of the soil orrock mass, possibly changing the values to be determined, however greatthe precautions taken.

Additionally, the volume of ground involved in the tests by the methodsnow in use is not as a rule sufficiently large to be representative ofthe properties of the whole. In practice, tests in which the loaded areaexceeds 1 square meter are too slow, and the area of loading isgenerally not more than 0.25 square meter, and, save in special cases,such an area is obviously insufficient.

An object of the invention is'to avoid the disadvantages indicatedhereinabove and to provide a new instrument by which soil deformabilitycan be determined under the following conditions:

the surfaces to be tested can be obtained by comparatively rapid andinexpensive methods;

test surfaces can be obtained which are entirely smooth, very extensiveand reaching to a considerable depth in the rock mass;

the flat jacks which apply the presure contain strainmeasuringinstruments and can be easily reused;

the rock mass is not disturbed when the slot is cut.

In accordance with a method of the present invention narrow slots aresuccessively cut by means of a disc with a diameter of, for example, 1m.

A slight overlap in the contiguous ends of successive slots makes itpossible to obtain a single slot with approximately the same length asthe sum of individual slots.

The depth of the slots can exceed the radius of the cutting disc, as acylindrical hole of suitable diameter is previously bored to accommodatea tubular support of the disc. This support, which acts as a guide forthe cutting operation, can be extended by means of similar units so thatslots can be cut to any depth.

In order to obtain a continuous surface for applying uniform pressureson the walls of the slot, these are extended by means of twocomparatively rigid semi-cylinders with adequate dimensions which areintroduced in the guide hole. It is only after this operation iscompleted that a flat jack containing strain measuring instruments andthe fiuid under pressure is introduced in the slot to displace the wallsof the slot.

From the values of the two parameters, pressure and displacement of theslot faces, it is possible by means of analytic or empirical formulas toobtain the modulus of deformability of the foundation mass.

If the distances between points, for instance at the surface, aremeasured before cutting the slot, the residual stress in the groundnormal to the plane of the flat jack is equal to the pressure requiredto cancel the change in the distance between the points observed duringthe cutting of the slot.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view of a cuttinginstrument;

FIG. 2 is a front view of the cutting instrument;

FIG. 3 is a side view of a flat jack with a shape suitable to completelyfill a slot which is cut in the material to be measured;

FIG. 4 is a side view of a -fiat jack with a different shape to be usedwhen the slot is not to be entirely filled; and

FIG. 5 is a sectional view showing the location of a strain measuringinstrument in the jack.

DETAILED DESCRIPTION The cutting apparatus comprises a supporting framehaving a base 1 bolted in the ground, two guiding columns 2 connected byan upper transverse element, and two E braces between the transverseelement and the base that increase the rigidity of the assembly.

Each of the two guiding columns 2 is formed with a rack 4 on a flat rearface. The rack 4 extends the entire length of the columns. A carrier 6can be moved vertically up and down along the guiding columns byoperation of a crank 5 which controls two pinions (not shown) whichengage racks 4.

Carrier 6 contains a gear box 7 operated by a motor 8. This can be anelectric, compressed air, or any other type of motor, although thehydraulic type is preferred due to its compact construction.

A cutting disc 9 is fitted on a shaft 10 which is mounted at the lowerend of a supporting tube 11. The shaft 10 and the disc 9 thereon, isdriven in rotation from the motor 8 through the gear box 7 and a doublechain (not shown) or other suitable power transfer means containedwithin tube 11.

A slot is progressively cut in the foundation mass to be tested by thedisc 9 which is advanced b the hand operated crank 5. Before cutting therock with the disc, a guide hole is drilled in the rock with asufficiently large diameter to permit subsequent guided passage of tube11 when the slot is cut by the disc. The bit for drilling the guide holemay be attached to the frame and operated by the same motor 8.

By means of a crank 12 and an eccentric 13, the tube 11 supporting thedisc can be given a lateral oscillatory movement which can be arectilinear or a pendulous movement. This facilitates the penetration ofthe disc in the rock mass, producing a slight enlargement in the lengthof the slot when the slot is being cut.

The tubular support of the disc 11 can be extended by means of similarelements fitted with suitable transmission systems so that its length isadapted to correspond with the desired depth of the slot.

After successive slots have been cut to form an elongated slot ofdesired length, a jack is inserted into the slot to apply a measuredpressure on the walls bounding the slot in order to measure thecorresponding deflection of the walls whereby the deformability of thefoundation mass can be measured.

The jacks by which uniform pressure is applied to the slot walls areessentially constituted of two flatted mildsteel flexible sheets 14welded along their edges to define an interior space. Any other highstrength very deformable material can be used instead of mild steel.

Flat jacks intended to fill the entire area of the slot have the shapeof a rectangle with a semi-circular end (FIG. 3). If the flat jacks neednot fill the entire area, they can be smaller and have other shapes,such as that shown in FIG. 4. The latter are used when the edges of theslots are not to be disturbed by applied forces.

Flat jacks of the former type include a flap 15 on the upper edge whichprotects the operators against projections of fluid under pressure incase of a rupture in the jack.

Both types of jacks are fitted with a nipple 16 for the inlet of fluidunder pressure and an outlet 17 for electric cables of the strainmeasuring instruments.

Each strain measuring instrument is applied at suitable points betweenthe faces of the flat jack, as shown in FIG. 5, and consists of twodeformable steel blades 18 each fixedly secured at its opposite ends toa respective plate. Electrical resistance strain gauges 19 are securedon the blades 18 and are incorporated in a Wheatstone bridge circuit andconnected with an external reading instrument,

4 where changes in the distance between the faces of the fiat jack canbe directly read.

It should be noted that the disclosed method and apparatus may bemodified without departing from the scope and spirit of the invention asdefined in the appended claims.

What is claimed is:

1. A method of measuring deformability and residual stress in afoundation mass, said method comprising cutting a plurality ofcontiguous narrow slots in a foundation mass so that a single elongatedslot is formed and applying forces to the faces bounding said elongatedslot to determine the deformability thereof.

2. A method as claimed in claim 1, wherein said forces are developed byinserting relatively displaceable plates into said elongated slot andapplying the plates with measuredforce against said faces.

3. A method as claimed in claim 2 comprising boring a hole in thefoundation mass prior to the cutting of said slots, and guiding thecutting of the slots by said hole.

4. A method as claimed in claim 1, wherein strains result in said massfrom said forces, comprising measuring the strains to determine theresidual stress in said mass.

5. Apparatus for applying forces to faces bounding an elongated slot formeasuring deformability and residual stress in a foundation mass, saidapparatus comprising a jack including two deformable flatted sheetshaving peripheral edges secured together to define an enclosed interiorspace, means for pressurizing said sheets against said faces, and meanswithin said space for measuring the displacement of the sheets as thefoundation mass deforms under the pressure applied to the sheets.

6. Apparatus as claimed in claim 5, wherein said sheets have a shapecorresponding to the entire face of the elongated slot.

7. Apparatus as claimed in claim 6, wherein said jack has an upper edgewith a protection flap thereat, said means for pressurizing said sheetscomprising an inlet at said flat edge for admission of a pressure fluidwhich is introduced into the space between said sheets, said means formeasuring the deformability of the foundation mass including electricalmeans within said space and an outlet at said flat edge.

8. Apparatus as claimed in claim 5, wherein said means for pressurizingsaid sheets comprises means for introducing a pressure fluid into thespace between the sheets.

9. Apparatus as claimed in claim 5, wherein said means in said space formeasuring displacement of the sheets comprises a flexible elementcoupled to both sheets to be stressed as the sheets undergo relativedisplacement, and strain measuring means mounted on said flexibleelement.

10. Apparatus as claimed in claim 5 in combination with means forcutting said elongated slot in the foundation mass, said jack being of asize operatively related to said cutting means for being insertable intosaid slot to apply forces to the faces thereof and measure deformabilitythereof.

References Cited UNITED STATES PATENTS 3,l42,173 7/1964 Pfann 7388.53,364,737 1/1968 Comes 73151 3,419,923 1/1969 Cowan 5348 JERRY W.MYRACLE, Primary Examiner U.S. Cl. X.R. 7384

